Flow rates and pressure

[mfgs]

Is there anyway of calculating what kind of flow rate and dynamic pressure increase you will get by upgrading the mains into a house?

At the moment I have a customer who wants an unvented cylinder. It has 4.7bar standing pressure, 25 lts/min flow rate and a dynamic pressure of 2 bar with kitchen tap open fully or 1.5 bar with a full bore 15mm valve opened into a bucket.

I am a little concerned this isnt going to be enough. At the moment there is just one bathroom with a shower, bath and basin. But in the future they want to be adding a couple of en-suite bathrooms. It currently has a 15mm main coming into the house, would the dynamic pressure be increased by upgrading to a 25mm main? Any way of calculating this?

 

[fuzzy]

what do you mean by dynamic pressure, not a term im familiar with?

 

[BLOD]

A 25mm main will give much better flow rate (not pressure).
The difference can be calculated but you need to know exactly what they are planning to put in.

 

[BLOD]

what do you mean by dynamic pressure, not a term im familiar with?

Dynamic pressure is working pressure as opposed to standing pressure

 

[mfgs]

A 25mm main will give much better flow rate (not pressure).
The difference can be calculated but you need to know exactly what they are planning to put in.

Surely by having a larger flow rate, less pressure will be lost with a tap open though? At the moment it drops from 4.7 to 1.5 from a full bore 15mm opening at 25 ltrs/min. So say you were to get 35 ltrs/min coming out of the same opening there would be less of a pressure loss? Or is my theory all wrong?

Does a 15mm supply not lose some of the working (dynamic) pressure due to friction compared to larger bore pipes?

 

[justlead1]

Hi 25 mm will be approx 3.8 time the performance of 15 mm

 

[BLOD]

A restricted supply Will decrease the flow and increase the pressure - a bit like putting your
thumb over the end of a hosepipe.

 

[AWheating]

you current figures should be adequate for a med unvented system. Adding flow restrictors into new fixtures will help ensure a more balanced system with less flow problems.

 

[mfgs]

Hi 25 mm will be approx 3.8 time the performance of 15 mm

In whar way exactly? Could you please explain it in a bit more detail, also how did you work it out?

Is there any way to increase the working (dynamic) pressure in a system then? Is there any relation to static pressure, dynamic pressure and flow rates? I dont know why I am struggling to get my head round this.

 

[mfgs]

A restricted supply Will decrease the flow and increase the pressure - a bit like putting your
thumb over the end of a hosepipe.

So by putting in a larger main I am going to lose dynamic pressure?

 

[mfgs]

you current figures should be adequate for a med unvented system. Adding flow restrictors into new fixtures will help ensure a more balanced system with less flow problems.

Cheers, I am confident it will work at the moment with just one bathroom. Its the addition of the en-suites in the future which concerns me. I dont want the customer forking out a load of money on something which wont work properly with what they have planned for a couple years down the line.

 

[justlead1]

Hi. I use Box's formula its as old as the hills but other than doing the job, theory and a bit of experience is the only way to calculate the outcome. There are quite a few things that effect delivery.
Mains pressure fluctuates on demand (peak times reduce to low levels in some communities)
Diameter of pipes, length of pipes, valves, taps, changes of direction. Plus the law of probability that suggests not all appliances will be on at the same time, which can be a bonus.
Wash hand basins and WC. cisterns rarely draw much attention with slow delivery, where as showers and baths are a different story. However i have found people weened on combi systems will be amazed with the performance of a middle of the road unvented system. You will also have to consider the storage capacity of the cylinder if sizing to give a large volume at delivery point, as cold water will follow. Its not an exact science as it may require educating clients as to expected performance. Good Luck

 

[mfgs]

Thanks justlead1. I am thinking a 210 litre tank should be ok for one main bathroom and a couple of en-suites in the future, havnt been able to find any kind of sizing guide anywhere though.

From your formula, is it just the flow rate that will increase from upgrading the mains. A better flow rate would help, but its mainly the drop in pressure from static to dynamic which is worrying me if 2 showers were on at the same time I think it may struggle.

 

[justlead1]

Hi. Shower heads vary massively in terms of discharge. As such they can be called part of the design. There are thousands of domestic properties with two showers or more that work perfectly well. Its not a one off, the concerns and awareness you have shown will result in a sound job i am sure. Good luck

 

[AWheating]

Thanks justlead1. I am thinking a 210 litre tank should be ok for one main bathroom and a couple of en-suites in the future, havnt been able to find any kind of sizing guide anywhere though.

From your formula, is it just the flow rate that will increase from upgrading the mains. A better flow rate would help, but its mainly the drop in pressure from static to dynamic which is worrying me if 2 showers were on at the same time I think it may struggle.

if you have 25lpm at 1.5 bar then you should easy run two mixer showers with 8lpm flow restrictors at the same time without pressure problems. Adding flow restrictors to other fixtures will ensure no dramatic shift in flow when additional fixtures run.

 

[AWheating]

at 210l cylinder will be more than adequate, most manufactures will provide tables/guide to enable you to decide. also follow manufactures guidelines to what kw is needed for the coil as they can vairy quite alot.

 

[mfgs]

Thanks for all the help guys. I am still wondering if I increase the main coming in, if it will help or not. If a larger pipe decreases the working pressure then I am not sure it would be wise. Is there any way of increasing the working pressure by altering pipework? I am really struggling to get my head around this for some reason.

 

[justlead1]

Mfgs. The pressure remains the same what ever size pipe when the water is at rest. but volume is needed to feed appliances. If you can find info. about hydrolic gradient it may help you focus. And get your head around the situation.